136 VEGETATION OF DRY REGIONS. [chap. 



which, as Tyndall has shown, the spray of perfume 

 possesses to bar out the passage of heat rays, and 

 has suggested that the emission of essential oils from 

 the leaves of many plants which live in hot climates 

 may serve to protect themselves against the intensely 

 dry heat of the desert sun. 



I am rather disposed to think that the aromatic 

 character of the leaves protects them by rendering it 

 less easy for animals to eat them. 



In still drier regions, such as the Cape of Good 

 Hope, an unusually large proportion of species are 

 bulbous. These, moreover, do not belong to any 

 single group, but are scattered among a large number 

 of very different families : the bulbous condition 

 cannot, therefore, be explained by inheritance, but 

 must have reference to the surrounding circumstances. 

 Moreover, in a large number of species the leaves 

 tend to become succulent and fleshy. Now in 

 organisms of any given form the surface increases as 

 the square, the mass as the cube, of the dimensions. 

 Hence, a spherical form, which is so common in 

 small animals and plants, and which in them offers a 

 sufficient area of surface in proportion to the mass, 

 becomes quite unsuitable in larger creatures, and we 

 find that both animals and plants have orifices leading 

 from the outside to the interior, and thus giving an 

 additional amount of surface. But in plants which 

 inhabit very dry countries it is necessary that they 

 should be able to absorb moisture when opportunity 

 offers, and store it up for future use. Hence, under 

 such circumstances fleshy stems and leaves are an 

 advantage, because the surface exposed to evaporation 



